Added inst-combine for 'MIN(MIN(A, 97), 23)' and 'MAX(MAX(A, 23), 97)'

This removes TODO added in r208849 [http://reviews.llvm.org/D3629]

MIN(MIN(A, 97), 23) -> MIN(A, 23)
MAX(MAX(A, 23), 97) -> MAX(A, 97)

Differential Revision: http://reviews.llvm.org/D3785

llvm-svn: 209110